Familial Mediterranean fever (FMF) is an autosomal recessive autoinflammatory disease characterised by paroxysmal attacks of serosal inflammation. Colchicine is highly effective in preventing these attacks but it may also disrupt the intestinal absorption of vitamin B12. We hypothesised that patients treated with colchicine for a prolonged period could develop deficiency of the vitamin. Ninety-five adult FMF patients on regular colchicine treatment for at least 2 years and age and sex-matched 90 healthy controls were enrolled and complete blood count with platelets, vitamin B12 and folic acid were measured in each person. We also investigated 15 adult FMF patients who were not yet on colchicine. The mean vitamin B12 values were not significantly different between the groups (352.12 (SD=171.62) pg/mL vs. 360.96 (SD=146.53) pg/mL, p=0.71), but there were significantly more vitamin B12 deficient cases among FMF patients (12 vs. 3; p=0.021) and 3 out of these 12 had megaloblastic anaemia. None of the vitamin B12 deficient controls had anaemia. We could not identify any disorder which might have causative effect for the deficiency among this subgroup. The mean vitamin B12 value of 15 colchicine-naïve cases was not significantly different from patients on colchicine (p=0.356). We did not observe significant vitamin B12 deficiency among colchicine-treated FMF patients but some cases may be more prone to developing this potentially serious disorder.